1. Field of the Invention
The present invention is directed to a developing apparatus used in an image forming apparatus in an electrophotographic method with one-component toner such as a copier, a facsimile, a printer, etc. More particularly, the present invention is directed to a developing apparatus which has a toner supporting member made of an elastic inner layer and a magnetic surface layer and in which non-magnetic portions are formed on both end portions of the magnetic surface layer.
2. Discussion of the Background
A photosensitive body of an image forming apparatus utilized in an electrophotographic method is categorized into a drum-type photosensitive body and a belt-type photosensitive body.
The drum-type photosensitive body is made of a relatively hard material. In a developing process, a toner supporting member for contacting the photosensitive body is made of an elastic material so as not to damage a photosensitive layer.
Japanese Patent Laid-Open Publications No. 3-259278, No. 3-259290, No. 4-181970, and No. 5-313474 disclose conventional toner supporting members in which a developing roller is used in an image forming apparatus. The developing roller has a magnetic surface layer with flexibility and an elastic inner layer.
In Japanese Patent Laid-Open Publications No. 3-259278 and No. 3-259290, the elastic inner layer has a rubber hardness based on JIS Japanese Industrial Standard Type A-K 6301 of 70.degree. C. or less.
In Japanese Patent Laid-Open Publication No. 4-181970, the magnetic surface layer is made of rubber into which magnetable particles are dispersed. A pitch between magnetic poles in the developing roller is 100 .mu.m or less.
In Japanese Patent Laid-Open Publication No. 5-313474, the developing roller has an inner surface made of foamed material and a surface layer with flexibility which is buried in the inner surface. Both end portions of the inner surface are not covered with the surface layer. The surface of the surface layer and the surface of the end portions in the inner layer are put on the same level. A thickness of the surface layer is 20.about.300 .mu.m.
In these devices, the developing roller and a toner shielding member are sufficiently and tightly contacted with each other so that toner is prevented from overflowing from end portions of the developing roller.
However, these devices have a drawback that frictional endurances of the surface layer and the inner layer are different from each other. As a result, the inner layer is worn down by friction of the toner shielding member first. A gap between the inner layer and the toner shielding member then grows. The result is that shielding effect is deteriorated and wearing waste particles are scattered and are attached to a photosensitive drum and an optical lens so as to deteriorate an image.
The following further drawbacks also result in the conventional devices.
The wearing waste particles are inserted between the developing roller and a developing blade so as to prevent the proper forming of a toner layer on the developing roller.
Foreign particles such as plastic or metal particles accidentally attached on the inner layer are brought into contact with the photosensitive drum, and a part of the photosensitive drum is scratched off so as to deteriorate image formation.
As the developing roller and the toner shielding member are tightly contacted with each other, a revolutional loading torque of the developing roller is large so that a motor for the developing roller is imposed with an overload.
As the surface layer is buried in the inner surface, a concave step is formed on the surface of the inner layer so that a process of mechanical forming and dimension control are complicated.